Evidence for the void size related IR absorption frequency shifts in hydrogenated amorphous germanium films

Hydrogenated amorphous germanium alloys (a-Ge:H) have been prepared by plasma enhanced chemical vapor deposition at a substrate temperature of 200°C. The films tend to oxidize as soon as they are taken out from the chamber and exposed to air. The oxidation process is found to follow a percolation channel formed by a continuous linkage of large voids that is evidenced by the observation of the decline of the intensity of Ge–H stretching mode at 1980 cm−1 to zero while the other peak at 1890 cm−1 does not change. This means that the peak at 1980 cm−1 corresponds to the Ge–H stretching mode in large voids while that at 1890 cm−1 corresponds to Ge–H stretching mode in isolated small voids. Thus, the frequency shifts of the stretching mode should arise from the difference of the void size where Ge–H bonds are situated.